1. Python Programming in Context Chapter 4

2. Objectives To understand Python lists To use lists as a means of storing data To use dictionaries to store associative data To implement algorithms to compute elementary statistics

3. Figure 4.1

4. List Heterogeneous collection of Python data objects Ordered Comma delimited inside square brackets [ ]

5. Figure 4.2

6. List Operations Concatenation Repetition Indexing Slicing Length

7. Mutable Lists are mutable Items can be changed by assignment Use index in left hand side of assignment statement

8. Figure 4.3

9. Figure 4.4

10. List Methods append insert pop sort reverse index count

11. Simple Statistics Compute simple statistics on a list of data Range Maximum, Minimum Mean, Median, Mode Standard Deviation

12. Listing 4.1 def getRange(alist): return max(alist)-min(alist)

13. Listing 4.2 def getMax(alist): maxSoFar = alist[0] for pos in range(1,len(alist)): if alist[pos] > maxSoFar: maxSoFar = alist[pos] return maxSoFar

14. Listing 4.3 def getMax(alist): maxSoFar = alist[0] for item in alist[1:]: if item > maxSoFar: maxSoFar = item return maxSoFar

15. Listing 4.4 def mean(alist): mean = sum(alist) / len(alist) return mean

16. Median Middle item Depends on length of list Even or Odd number of items

17. Figure 4.5

18. Listing 4.5 def median(alist): copylist = alist[:] #make a copy using slice operator copylist.sort() if len(copylist)%2 == 0: #even length rightmid = len(copylist)//2 leftmid = rightmid - 1 median = (copylist[leftmid] + copylist[rightmid])/2 else: #odd length mid = len(copylist)//2 median = copylist[mid] return median

19. Dictionary Collection of associated key-value pairs Fast lookup Comma delimited key:value pair in curly braces { } Use index operator and key to look up value Use it to implement item counting

20. Figure 4.6

21. Dictionary Methods keys values items get

22. Listing 4.6 def mode(alist): countdict = {} for item in alist: if item in countdict: countdict[item] = countdict[item]+1 else: countdict[item] = 1

23. Listing 4.7 def mode(alist): countdict = {} for item in alist: if item in countdict: countdict[item] = countdict[item]+1 else: countdict[item] = 1 countlist = countdict.values() maxcount = max(countlist) modelist = [ ] for item in countdict: if countdict[item] == maxcount: modelist.append(item) return modelist

24. Listing 4.8 def frequencyTable(alist): countdict = {} for item in alist: if item in countdict: countdict[item] = countdict[item]+1 else: countdict[item] = 1 itemlist = list(countdict.keys()) itemlist.sort() print("ITEM","FREQUENCY") for item in itemlist: print(item, " ",countdict[item])

25. Figure 4.7

26. Listing 4.9 def frequencyTableAlt(alist): print("ITEM","FREQUENCY") slist = alist[:] slist.sort() countlist = [ ] previous = slist[0] groupCount = 0 for current in slist: if current == previous: groupCount = groupCount + 1 previous = current else: print(previous, " ", groupCount) previous = current groupCount = 1 print(current, " ", groupCount)

27. Drawing a Frequency Chart Use turtle to draw a picture of the data Use frequency count data

28. Figure 4.8

29. Listing 4.10 part 1 import turtle def frequencyChart(alist): countdict = {} for item in alist: if item in countdict: countdict[item] = countdict[item]+1 else: countdict[item] = 1 itemlist = list(countdict.keys()) minitem = 0 maxitem = len(itemlist)-1 countlist = countdict.values() maxcount = max(countlist) wn = turtle.Screen() chartT = turtle.Turtle() wn.setworldcoordinates(-1,-1,maxitem+1,maxcount+1) chartT.hideturtle()

30. Listing 4.10 part 2 chartT.up() chartT.goto(0,0) chartT.down() chartT.goto(maxitem,0) chartT.up() chartT.goto(-1,0) chartT.write("0",font=("Helvetica",16,"bold")) chartT.goto(-1,maxcount) chartT.write(str(maxcount),font=("Helvetica",16,"bold")) for index in range(len(itemlist)): chartT.goto(index,-1) chartT.write(str(itemlist[index]),font=("Helvetica",16,"bold")) chartT.goto(index,0) chartT.down() chartT.goto(index,countdict[itemlist[index]]) chartT.up() wn.exitonclick()

31. Standard Deviation Accumulator pattern Math module Sum of squares

32. Listing 4.11 import math def standardDev(alist): theMean = mean(alist) sum = 0 for item in alist: difference = item - theMean diffsq = difference ** 2 sum = sum + diffsq sdev = math.sqrt(sum/(len(alist)-1)) return sdev